Thesiger attended Marlborough College and the Slade School of Art with aspirations of becoming a painter, but quickly switched to drama, making his professional debut in a production of Colonel Smith in 1909.
After the outbreak of World War I, on 31 August 1914 Thesiger volunteered as Rifleman No.2546 with the 2nd Battalion of the 9th London Regiment (Queen Victoria's Rifles), T.F. at its Regimental Head Quarters in London's West End. After training in England for 3 months he was sent to the Western Front in late 1914, and was wounded in the trenches on 1 January 1915,[3] and medically evacuated back to England.[4] At a dinner party shortly after his return, someone asked him what it had been like in France, to which he is supposed to have responded "Oh, my dear, the noise! and the people!"
In 1917, he married Janette Mary Fernie Ranken (1877-1970), sister of his close friend and fellow Slade graduate William Bruce Ellis Ranken. In her biography of Thesiger's friend, Ivy Compton-Burnett, Hilary Spurling suggests that Thesiger and Janette wed largely out of their mutual adoration of William, who shaved his head when he learned of the engagement. Another source states more explicitly that Thesiger made no secret of his homosexuality.[5]

Thesiger moved in several artistic, literary and theatrical circles. At various times, he frequented the studio of John Singer Sargent, befriended Mrs. Patrick Campbell, visited and corresponded with Percy Grainger and worked closely with George Bernard Shaw, who wrote the role of the Dauphin in Saint Joan for him. Somerset Maugham, on the other hand, responded to Thesiger's inquiry as to why he wrote no parts for him with the quip, "But I am always writing parts for you, Ernest. The trouble is that somebody called Gladys Cooper will insist on playing them."

Thesiger's film debut was in 1916 in The Real Thing at Last, a spoof presenting Macbeth as it might be done by an American company, in which he did a drag turn as one of the Witches.[6] Thesiger also played the First Witch in a 1941 production of Macbeth directed by John Gielgud. He performed more small roles in films during the silent era, but worked mainly on the stage.

In 1925, Thesiger appeared in Noël Coward's On with the Dance, again in drag, and later played the Dauphin in Shaw's Saint Joan. He wrote an autobiography Practically True, published in 1927, which covers his stage career. An unpublished memoir written near the end of his life is housed in the Ernest Thesiger Collection at the University of Bristol Theatre Collection.

When Whale agreed to direct Bride of Frankenstein in 1935, he insisted on casting Thesiger as Dr. Septimus Pretorius, instead of the studio's choice of Claude Rains. Partly inspired by Mary Shelley's friend John Polidori and largely based on the Renaissance physician and botanist Paracelsus, it became Thesiger's most famous role, in which he gives a fey, flamboyant performance as Baron Frankenstein's mentor.

Arriving in the United States for the filming of Bride of Frankenstein, Thesiger immediately set up a display in his hotel suite of all his needlework, each with a price tag, and during the making of the film he would work on needlework, one of his hobbies.[7]

The remainder of Thesiger's career was centered on the theatrical stage, though he did appear in supporting roles in films produced in Britain, prominent among which is The Man in the White Suit (1951), starring Alec Guinness. He plays "Sir John," the most powerful, the richest, and the oldest of the industrialists (jointly with the trade unions) trying to suppress Guinness's invention of a fabric that never wears out and never gets dirty.[8]

In 1960, Thesiger was awarded the Commander of the Order of the British Empire (CBE). His last film appearance was a small role in The Roman Spring of Mrs. Stone (1962). Shortly after completing it, Thesiger died in his sleep from natural causes on the eve of his 82nd birthday, and is buried in Brompton Cemetery, London.[9]

In the fictionalised James Whale biopic Gods and Monsters (1998), Thesiger was portrayed by Arthur Dignam. And the real Thesiger is seen in the film when Brendan Fraser, as Whale's gardener, sits at a bar watching televised repeats of the original 1935 Bride of Frankenstein.